There is increasing interest in developing wideband and/or multiband antenna systems for use in wireless communications, microwave tomography, remote sensing, and other applications. The demand for high channel capacity (high data rate) is rapidly increasing because high data rate is required for multiple functionalities like browsing the internet, video streaming, online gaming, and on-road navigation. The next generation wireless standard will provide an increase in the overall channel capacity 1,000 times greater than current capacity, with multi-Giga bits per second expected to be a reality by the year 2020.
The multiple-input-multiple-output (MIMO) technology will therefore serve as a key enabling factor in achieving such high data rates. These antennas will cover different frequency bands of different standards and will support high data rates. Many portable devices have now multiple functionalities as compared to early generations with the existence of multiple antennas. Depending upon the size and targeted application, the user terminal will be allowed to carry up to 8 antennas with a minimum of 4 antenna elements.
Future wireless standards will rely on novel technologies to increase the data rates and provide reliable links. Current fourth generation (4G) and upcoming 5G will rely on multiple antenna systems with multi-standard support. These multiple standards will operate in different frequency bands with enough frequency bandwidth to provide the expected high data throughput. Antenna elements are usually isolated from one another, and thus occupy a large space within a wireless terminal. The concept of connected arrays (CA) was recently introduced for single band coverage and with single arrays. Cell phones will have elements that are of smaller size and maybe less efficiency than tablets that have more real estate to have more efficiency antenna systems.
The use of multiple-input multiple-output (MIMO) technology as well as the use of higher frequency bands beyond those currently used for wireless communications (i.e. above 6 GHz) will be key factors in achieving the throughput increase. The user terminal will be allowed to carry up to 8-antenna elements within current cellular bands below 6 GHz, with a minimum of 4-antenna elements, depending on the device size and application.
Integrating higher frequency band antennas or antenna arrays along with MIMO antenna systems at the lower bands will be a must to satisfy the large increase in the data throughput expected, as bandwidths of at least 500 MHz are required, and these are not available in the lower spectrum bands.
Such integrated antenna systems that support multiple antennas as well as multiple standards with capabilities both less than 6 GHz and above 10 GHz are of extreme importance for upcoming wireless handheld devices to be able to achieve the expected performance of 5G standards.
Due to the use of multiple antennas in MIMO configurations, space becomes an issue, especially for lower frequency bands, as the antenna elements become larger in size. Coming up with novel compact size and highly efficient antennas is very desirable. At higher frequency bands, i.e. higher than 10 GHz, the free space attenuation of the radio signals becomes large, and thus antenna array configurations are preferred to provide higher gains and compensate for such losses.
Designing a novel, compact size, directional MIMO antenna system with high gain, high isolation and low correlation between the MIMO channels is of great value because they become compatible with multiple standards and simultaneously cover multiple bands without the need of extra hardware for reconfigurability or frequency switching. Directional radiation characteristics, along with wide bandwidth and high efficiency, are required for good MIMO performance, as directional patterns mean more isolated channels and thus better performance and low inter-element correlation. Therefore, there is high interest in using directional antennas like Yagi-Uda in future 5G technology.
Yagi-Uda antennas are well known for their highly directional radiation patterns, high FBR, high gain, low cross polarization, controllable input impedance, and moderate bandwidth that can be increased. Yagi antennas are highly compatible with printed RF circuitry because they are robust and can be easily fabricated. However, the main challenge faced in designing Yagi antennas is their large size due to the presence of the large ground plane or number of reflector elements required to achieve high 1-BR, and the large number of director elements required to achieve high directivity. Hence, such antenna systems are not suitable to be used in small form factor wireless devices due to the limited space available. Despite the distinct features of such antennas, the size issue limits their use in modern small user terminals.
Accordingly, there is need for a highly miniaturized, compact size, low profile, Yagi-based MIMO antenna system for small form factor devices including mobile phones and other compact wireless devices, wherein the antenna system has wide bandwidth, high directivity and high efficiency and satisfies both fourth generation (4G) and 5G wireless communication bands.